A local-area network ("LAN") is a communication system that enables; personal computers, work stations, file servers, repeaters, data terminal equipment ("DTE"), and other such information processing equipment located within a limited geographical area such as an office, a building, or a cluster of buildings to electronically transfer information among one another. Each piece of information processing equipment in the LAN communicates with other information processing equipment in the LAN by following a fixed protocol (or standard) which defines the network operation. Information processing equipment made by different suppliers can thus be readily incorporated into the LAN.
The ISO Open Systems Interconnection Basic Reference Model defines a seven-layer model for data communication in a LAN. The lowest layer in the model is the physical layer which consists of modules that specify (a) the physical media which interconnects the network nodes and over which data is to be electronically transmitted, (b) the manner in which the network nodes interface to the physical transmission media, (c) the process for transferring data over the physical media, and (d) the protocol of the data stream.
IEEE Standard 802.3, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, is one of the most widely used standards for the physical layer. Commonly referred to as Ethernet, IEEE Standard 802.3 deals with transferring data over twisted-pair cables or co-axial cables which are typically more expensive than twisted-pair cables. The 10 Base-T protocol of IEEE Standard 802.3 prescribes a rate of 10 megabits/second ("Mbps") for transferring data over twisted-pair cables.
Referring to the drawings, FIG. 1 illustrates a typical example of how a conventional 10 Base-T media-access unit ("MAU") interfaces with an outgoing twisted-pair copper cable 10T and an incoming twisted-pair copper cable 10R. The 10 Base-T MAU in FIG. 1 is part of a personal computer, a work station, a file server, a bridge, a repeater, or DTE. The MAU consists of an interface controller 12, a 10 Base-T transceiver 14, two sets of terminating resistors 16T and 16R, two low-pass filters 18T and 18R, two isolation transformers 20T and 20R, two common-mode chokes 22T and 22R, and an RJ-45 cable connector 24, all situated on an Ethernet adapter card (not indicated). Twisted-pair cables 10T and 10R are part of a larger cable having a plug (not shown) that plugs into RJ-45 connector 24 along an edge of the adapter card.
Interface controller 12 controls the transmission of outgoing data to cable 10T and the reception of incoming data from cable 10R. 10 Base-T transceiver 14 converts non-differential outgoing data from controller 12 into differential form represented by signals TXO+ and TXO-. The differential outgoing data moves through resistors 16T, filter 18T, transformer 20T, and choke 22T where, in modified differential form represented by signals TX+ and TX-, the outgoing data is supplied through RJ-45 connector 24 to outgoing cable 10T as a data stream moving at the 10 Base-T rate of 10 Mbps.
Before being furnished to outgoing copper cable 10T, the outgoing data is Manchester encoded to reduce electromagnetic interference ("EMI"). The Manchester encoding causes some portions of the data stream to be pulses at 10 MHz while other portions are pulses at 5 MHz. In low-pass filtering the outgoing data, filter 18T typically removes frequency components above approximately 15-20 MHz. This is below the 30-MHz frequency above which the Federal Communications Commission ("FCC") places controls on electromagnetic radiation from unintentional radiators.
Differential incoming data on incoming copper cable 10R is supplied through connector 24 as signals RX+ and RX-. The incoming data passes through choke 22R, transformer 20R, filter 18R, and resistors 16R where, in modified differential form represented by signals RXI+ and RXI-, the incoming data is furnished to transceiver 14 for conversion to non-differential form and transfer to controller 12.
The ever growing need to transfer more information faster, accompanied by increases in data processing capability, is necessitating an expansion to data transfer rates considerably higher than the 10-Mbps rate prescribed by the 10 Base-T protocol. As a consequence, a protocol referred to as 100 Base-TX is being considered for extending IEEE Standard 802.3 to accommodate data moving at an effective transfer rate of 100 Mbps through twisted-pair cables of presently existing types.
Under the proposed 100 Base-TX protocol, certain control bits are incorporated into the data before it is placed on a twisted-pair cable. The result is that the data and control signals actually move through a twisted-pair cable at 125 Mbps. The 125-Mbps cable rate corresponds to a maximum pulse frequency of 62.5 MHz. Since this exceeds the 30-MHz frequency above which the FCC requires that EMI be controlled, the 100 Base-TX protocol specifies that the data be scrambled and provided with a trinary MLT-3 (multilevel transmit/three levels) coding before entering a twisted-pair cable. Although MLT-3 coding has 6 dB less noise immunity than binary coding for equal transmit signal amplitudes, MLT-3 has less conducted power at the key frequencies of 31.25 MHz and 62.5 MHz than binary coding and therefore has less EMI at these frequencies.
In expanding IEEE Standard 802.3 to the 100 Base-TX protocol, there will be various situations in which it is desirable that the physical transmission media be capable of handling data transferred through twisted-pair cables at both the 100 Base-TX rate and the lower 10 Base-T rate. Accordingly, a user-friendly apparatus that can transfer data at both rates is needed.
In particular, a person using information processing equipment capable of handling data moving through twisted-pair cables at either the 10 Base-T rate or the 100 Base-TX rate should not have to throw a switch, or make another such physical adjustment, when the data transfer rate changes from 10 Base-T to 100 Base-TX and vice versa. Also, when connecting a twisted-pair cable to data transfer apparatus, the user should not have to make accommodations depending on whether the cable comes from equipment that can handle data moving at the 10 Base-T rate, at the 100 Base-TX rate, or at both rates. To keep the cost low, it is desirable to use a small number of items such as isolating transformers, which are relatively expensive, and cable connectors in transferring data at both rates.